Phosphorescent materials



Patented Aug. 9, 1949 PHO SPHORESCENT MATERIALS Charles L. Graham and Herbert J. Dietz, Rochester, N. Y., assignors to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey No Drawing. Application June 10, 1944, Serial No. 539,792

4 Claims. 1

This invention relates to a method of preparing phosphorescent screens containing zinc sulfide.

While we have found our improved methods particularly useful in the making of phosphorescent screens to be used in a reflex copying process, such for instance as that disclosed in U. S. Patent Christensen, No. 1,565,256, granted December 15, 1925, they are by no means limited to that purpose. That process comprises exposing a phosphorescent screen to an exciting light and placing the glowing screen in contact with a document to be copied, at the same time exposing it through the back to red light which quenches the excitation except at the points opposite a black or non-reflecting portion of the document.

The screen may be made by dispersing the phosphorescent pigment in a vehicle such as a cellulose ester and coating the dispersion upon a suitable support. Such a screen should produce copies of good contrast and definition upon a sensitive material placed in contact with it, and we have found that this property is enhanced when the grain size of the pigment is relatively small, with an average diameter of from 10 to 15 mu. The grains must be thoroughly dispersed in the medium and the layer of phosphorescent material, and any transparent overcoating must be very thin.

However, prolonged grinding such as is usually necessary to disperse the pigment thoroughly and to reduce its grain size impairs its phosphorescent properties. The use of surface-active dispersing without grinding does not produce the desired fine grains, and the amount of active grain surface is relatively small.

It is to be understood that we use a zinc sulfide previously subjected to prolonged heat treatment at high temperature, say for two hours, at a temperature of the order of 1700 F., and which is in the form of relatively coarse grains having high phosphorescent properties. This product is obtainable on the market.

We have found that if the phosphorescent, large-grained, zinc sulfide is first dispersed. in a solvent in the presence of a long-chain fatty acid, such as stearic acid, followed by the heating and. removal of the solvent, its phosphorescent properties will be retained after a reasonable amount of grinding and its dispersibility increased by the heating operation, and the particles may be readily and uniformly dispersed by a final grind" ing operation in a coating composition which permits the formation of a very thin coating.

A detailed description of the entire process follows. The relatively coarse phosphorescent grains of zinc sulfide are included in the following dispersion:

Grams Zinc sulfide 910 Stearic acid 20 Butyl acetate 200 This mixture is ground in a pebble mill for 1 to 2 hours. After settling, the supernatant liquid is decanted and the remainder heated to 70 C. for several hours, at least three hours and preferably six hours or more, during which time the solvent evaporates completely. 7

At this point it should be pointed out that the purpose of this heating is not to remove the solvent, since the same solvent may be present in the coating composition in which the phosphorescent grains are later dispersed. There appears to be some chemical action, the mechanism of which'is not clear, between the surface of the zinc sulfide grains and the higher fatty acid, which takes place as a result of the heating operation. Whatever the theory of the process, we have discovered and claim as an essential feature of our invention the surface treatment of the zinc sulfide grains with a higher fatty acid with heating. Presumably the stearic acid adsorbed to or reacted with the fresh surfaces, presumably forming zinc stearate.

It should also be pointed out that the temperature of heating is about the melting point of stearic acid. We contemplate the use of other higher fatty acids, and in such cases the temperature of heating should be at about the melting point of the acids used. In the specific example given the temperature mentioned is just above the melting point of stearic acid, and no useful results are obtained by using a higher temperature, but the action takes place to the required extent at temperatures above and somewhat below the melting point, the lower limit of usefulnessbeing at about 62, and the recom mended lower limit being about 65 C. When in the claims We refer to' a range of the order of the 3 melting point, we mean the useful range as here given. The length of heating time is not at all critical, but should be at least three hours to obtain markedly useful results.

The powder resulting from the above operations is then dispersed in the following composition:

Grams Cellulose nitrate (high viscosity) 36.3 Butyl acetate 418.0 Toluene (or benzene) 273.0 Butyl phthalate 18.0

Dispersion is accomplished by grinding for an hour or two in a pebble mill, and the screens are then coated from this composition.

The total time of grinding in the two operations is preferably not more than three hours under ordinary grinding conditions, sinc'e 'extended grinding reduces the phosphorescent properties. In that time the grains are reduced from an irregular grain size including grains as large as .040 mm. to a fairly uniform grain'size of from 0.010 to 0.015 mm. The optimumgrinding time cannot be categorically stated "for all conditions, asit will depend on the size of the mill,*size and numberof pebbles, size ofload,"speed of mill, and other'factors affectingthe violence of the grinding operationbutin 'order'to' produce thin compact layers we find that the ratio of zinc sulfide to cellulose ester must' be 'high,'of theor'der of 25 to 1. A compositionw'hich will have a'suitable coating viscosity'should include solvents and diluents producing a high viscosity with a low solid icontent, the cellulose ester being about 5 per cent of the composition, exclusive 'of the phosphorescent pigment. The formula given above is by way of example, it being understood that otherplastics and particularly other cellulose esters and derivatives'such'as ethyl cellulose may be used.

The screen may "bema'd'e by first coating ona sheet of glass a very thin'transparentlayer-over which the second or phosphorescent layer 'is formed, and over this in turn is formed a third or transparent supporting layer which may be colored with a dye transmitting red light. These three coatings'areto'gether stripped from the plate and torm "the complete screen, the first layer acting as a protective layer.

This first layer may be coated from a lO'per cent solution of'cellulose'acetate in acetone, the coating being 00.12 mm. thick when poured and drying to 00.01 mm. The screen should have the best possible contact-With b'othth'e document to be copied and also with the sensitive'material, and it is therefore important-th'at the protective layer be very thin.

The phosphorescent composition is 'coated'to a thickness of about .2 mm. 'andwhendry'ha'sa thickness from .08 mm. to .11 mm.

The thickness and composition-of the support is relatively immaterial so far as the invention here disclosed is concerned. It has hitherto been impossible to obtain a fine grained "zinc sulfide screen of high phosphorescentproperties in a continuous layer'as thin as 0.1 mm.

Since the luminescent light is emitted'from one surface of the screen, and the quenching light enters from the other surface, the screens should be as thin and at the same time as uniformly luminescent as possible, and these desid'erata are obtained by our process.

The phosphorescentscreen so produced e'mits green light with a'maximum at530mil1imicrons. It is excited by blue and violet light and quenched in a phosphorescent screen that comprises grinding zinc sulfide in the form of coarse grains resulting froznheat treatment and dis. rsing the .grainsina coating composition, Chara prized by grinding the coarse grains in the presence of stearic acid in solution for at least an hour and until the grains are thoroughly dispersed, then removing theliquid and maintaining the grains at-a temperature between 62 and C. for at least three hours.

2. "The method of preparing zinc sulfide for use in a phosphorescent screen that comprises grinding zinc sulfide, in the form of coarse grain-s resulting from heattreatment and having high phosphorescent properties, in an inert liquid containing stearic acid in solution until the grains 1 are thoroughly dispersed, permitting the grains to settle, removing the supernatant liquid, and heating the residue at a temperature between 65 and 75C. for at least three hours, whereby the solvent is evaporated, the particle size is reduced, and the stearic acid reacts-With the surfaceof the particles.

3. The method of preparing a phosphorescent screen that comprises'grinciing zinc sulfide, in the form of coarse grains resulting from heat treatment and havinghighphosphorescent properties, in an inert-liquid'containing stearic acid insolution until the grains are thoroughly dispersed, permitting the grains to settle, removing the supernatant liquidyheating the residue at a temperature between62 and 75 C. for at least three hours, and thengrinding theparticles ina coatingcompositiomof which-5 per cent is a cellulose ester, and forming-a coating from said composition.

4. Themethod of preparing a phosphorescent screen that comprises grinding zinc sulfide, in the form of coarse grainsresulting from heat treatment and having high phosphorescent properties, in an inert liquid containing stearicacid in solution until the .grains are thoroughly dispersed,'permitting the grains to settle, removing the supernatant liquid, heating the residue at a temperature between-65 and 75 C. for at least three hours, and-thengrinding the particles in a coating compositiomof which 5 per cent is cellulose ester, and the relative proportions of zinc sulfide to cellulose esteris'of the order of 25 to 1, and forming -a "coating from said composition, the-total time of grinding in both operations being not over three hours.

CHARLES L. GRAHAM. HERBERT J. DIETZ.

REFERENCES CITED The following referenices are of record in the file of thispatent:

UNITED STATES PATENTS Number Name Date '1',532,782 Shappard Apr. 7, 19.25

(Othcrreference'son following page) 6 UNITED STATES PATENTS Number Name Date Number Name Date 2,274,766 Ziehl Mar. 3, 1942 1,569,484 Hall Jan. 12, 1 26 2,310,424 Goodman Feb. 9, 1943 1,947,504 Starke Feb. 20, 1 34 2,317,977 Cassellini May 4, 1943 1,954,462 Tainton Apr. 10, 1934 2,366,270 Lemmers Jan. 2, 1945 1,985,076 Breyer Dec. 18, 1334 2,395,185 Isenberg Feb. 19, 1946 2,002,891 Hall Ma 28, 1 5

2,103,032 Fritze Dec? 21, 1937 OTHER REFERENCES 2,177,691 Dawihl et a1 Oct. 31, 1939 De Ment Fluorochemistry, published 1945 by 2,101,305 Myers Nov. 28, 1939 10 Chemical Publishing 00., pp. 202 and 359. 2,202,048 Einig et a1 May 28, 1940 

